The present invention relates to a dry type development apparatus for developing latent electrostatic images in facsimile apparatus and electrophotographic copying apparatus employing powder developer containing a magnetic material.
As a dry type development apparatus, the so-called magnetic brush development apparatus is known, which comprises a non-magnetic sleeve with internally disposed magnets. In such a development apparatus, a magnetic brush made of a developer containing a magnetic material is formed on the surface of a non-magnetic sleeve and latent electrostatic images are developed by moving the magnetic brush in contact with the latent electrostatic images.
When the latent electrostatic images are developed by the development apparatus of this type, it is required that the gap between the tip of the magnetic brush and a latent image bearing member, and the time the magnetic brush is in contact with the latent image bearing member, be kept constant respectively. Otherwise, the developed visible images may become uneven in image density and the developer may be deposited on the background of the developed images. When the latent electrostatic image bearing member is made of a rigid member such as a photoconductor drum and is rotated at a predetermined speed, comparatively it is not so difficult to prevent the above-mentioned undesirable phenomenon, since the gap between the tip of the magnetic brush and the surface of the photoconductor drum can be maintained nearly constant, for instance, by regulating the height of the magnetic brush by using a doctor blade.
However, in the case of a flexible latent electrostatic image bearing member, such as a recording paper to be used in a signal reception section of facsimile apparatus, it is extremely difficult to maintain the gap between the top of the magnetic brush and the surface of the flexible latent image bearing member constant. Furthermore, in the facsimile apparatus, the transportation speed of the recording paper is not always constant. In other words, the recording paper may be transported continuously or intermittently, depending upon the density of information to be received. Therefore, the time the magnetic brush is in contact with the recording paper may not be constant, and the recording paper may not always be moved in the same direction, either.
More specifically, the time the recording paper in the facsimile apparatus is in contact with the magnetic brush varies. This may cause uneven development of latent electrostatic images, fog of recorded images and smearing of the background of the recording paper.
In order to eliminate the above-mentioned problems, it could be proposed that the magnetic brush be moved in conformity with the movement of the recording paper so that the time the latent electrostatic images and the magnetic brush are in contact with each other is kept constant, or it could be proposed that, after development, the recording paper is cleaned by a cleaning apparatus to remove the developer deposited on the background of the recording paper. The former method, however, cannot be performed easily by use of a conventional magnetic brush apparatus, since a complicated mechanism is required to carry out the method, and the latter method has a disadvantage that the apparatus for performing the methods tends to become oversized and expensive, since it requires a cleaning apparatus in addition to the development apparatus.
In short, in the development apparatus of facsimile apparatus, smearing of the background of the recording paper, fog of recorded images, and uneven development cannot be overcome by simple use of the conventional techniques.
Referring to FIG. 1, there is shown a conventional development apparatus having means for keeping the gap between a flexible latent image bearing member and a non-magnetic sleeve constant. In FIG. 1, reference numeral 1 represents a non-magnetic stationary sleeve. Inside the sleeve 1, there is rotatably disposed a magnetic roller 2, which is alternately magnetized to negative and positive polarity in the circumferential direction of the sleeve 1. The magnetic roller 2 is rotated in the direction of the arrow by a suitable drive means (not shown). Under the sleeve 1, there is disposed a toner container 3 for holding a one-component type magnetic developer T therein. When the magnetic roller 2 is rotated clockwise, the toner T placed in the toner container 3 is moved counterclockwise along the surface of the sleeve 1, so that the toner T attracted magnetically to the surface of the sleeve 1, in the form a brush, is regulated by a doctor blade 4 in a manner such that the height of the brush is constant.
A gap roller 5 is rotatably disposed near the sleeve 1, with a predetermined space therebetween. The gap roller 5 is made of an electrically conductive material. In this development apparatus, the gap roller 5 serves to guide a recording paper 6 of a facsimile transceiver. Latent electrostatic images are formed on the recording paper 6 by a latent electrostatic image formation apparatus 7.
The recording paper 6 is transported in the direction of the arrow by a paper transport apparatus (not shown). During the transportation of the recording paper 6, the toner T on the surface of the sleeve 1 is supplied to the latent electrostatic images formed on the recording paper 6. The gap between the recording paper 6 and the surface of the sleeve 1 is maintained constant by the gap roller 5.
In the development apparatus as shown in FIG. 1, it is required that the distance between the sleeve 1 and the gap roller 5 be maintained small and, at the same time, the sleeve 1 and the gap roller 5 be held axially parallel to each other. However, this is very difficult in practice, since their respective adjustable ranges are narrow and in order to compensate for the narrow adjustable ranges, the accuracy of finish of each of the other members, such as support members of the gap roller 5, has to be increased and this is very expensive. Furthermore, this apparatus has a shortcoming that non-uniform development and smearing of the background are apt to occur when the recording paper 6 is transported intermittently.
More specifically, if the recording paper 6 is stopped while the toner T is present on the surface of the sleeve 1, a portion of the recording paper 6, which is nearest the toner T, will attract multiple layers of the toner T thereto, so that image density of the portion will become higher than that of the other portions of the recording paper 6. As a result, the image density of the recording paper 6 becomes non-uniform. When the above-mentioned portion of the recording paper 6 happens to be a non-image area, the toner T will deposit on the portion, so that the background of the non-image area is smeared by the toner T.